In conformity with the development of techniques in aircraft industry, motorcar industry and various other industries, and particularly in conformity with necessity of saving energy and resources cried for recently, there have early been sought (1) carbon fibers having high strength and modulus of elasticity which are usable for the production of light weight composite materials and (2) moldable carbon materials having high strength and modulus of elasticity which are usable for various purposes after compression molding. The present invention relates to a process for producing such a material suitable for the production of carbon fibers and moldable carbon materials, i.e., a homogeneous mesophase pitch having a low softening point that is moldable by, for example, melt spinning at relatively low temperatures.
The meaning of the term "mesophase" is not necessarily standardized in the academic world or various technological literatures. The term "mesophase" herein indicates an optically anisotropic portion which is one of the constituents of pitch. If the section of a pitch mass solidified at a temperature around room temperature is polished and observed by means of a reflected polarized light microscope under crossed polarizer and analyzer, a sheen is observed under stage rotation which is an optically anisotropic portion of the pitch. An optically isotropic portion of the pitch is that in which no sheen is observed with the operation mentioned above, and the isotropic portion will be called "non-mesophase" hereinafter.
Generally, when heavy hydrocarbons such as tar and pitch, which originally are in completely non-mesophase states are heat-treated to affect thermal cracking and polycondensation reactions, spherules of mesophase begin to appear in the pitch, which spheres grow gradually by coalescence. As compared with the non-mesophase portion, the mesophase comprises mainly molecules of a chemical structure in which polycyclic aromatic condensed rings have much more developed planar structure and orientation and in which the molecules are cohesively associated together to form a laminate of the planes. When molten, the mesophase has optical properties associated with crystals and hence mesophase is considered a liquid crystal state. If mesophase pitch is spun by extrusion through a thin nozzle, the planes of the molecules are arranged nearly along the axis of the fiber. Therefore, the carbon fibers made of the mesophase each have a high modulus of elasticity.
The amount of mesophase in a pitch is determined by polarized light microscopic examination of polished samples by relating the area of the optically anisotropic portion to the total area examined. The result is expressed as volume %. A pitch comprising mainly mesophase, and less than 10% non-mesophase, is called "mesophase pitch" herein.
As for the homogenicty of pitch, a pitch having a mesophase content in the range of about 90% to 100%, determined as above, and containing infusible particles (particle diameter of at least 1.mu.) which are practically undetectable in the micrographic observation, is herein called "substantially homogeneous mesophase pitch", since it exhibits an excellent homogeneity in the actual melt spinning process.
The term "softening point" of the pitch herein indicates a temperature at which the pitch is converted from the solid to the liquid phase. This temperature is the temperature at the peak of the absorption and release of latent heat when the pitch is fused or solidified and is determined with a differential scanning calorimeter. This temperature coincides with that determined by another method (such as ring-and-ball method or micromelting point method) with an error within plus or minus 10.degree. C. The term "low softening point" herein indicates a softening point in the range of about 230.degree. C. to 320.degree. C.
Several processes have been proposed for the production of mesophase pitch required for the production of high-performance carbon fibers. However, those processes have many problems such as those shown below:
(1) The starting materials are commercially not easily available.
(2) A reaction for a long period of time is required or complicated steps are required.
(3) Production costs are high.
(4) As mesophase is increased to close to 100%, the softening point is elevated to make the spinning difficult.
(5) If the softening point is controlled, the pitch becomes heterogeneous and the spinning thereof becomes difficult.
More particularly, a process disclosed in the specification of Japanese Patent Publication No. 8634/1974 necessitates (a) a starting material which is unavailable in a large amount at a low cost such as chrysene, anthracene or tetrabenzophenazine, (b) complicated production steps including dry distillation of a tar obtained by cracking a crude oil at a high temperature followed by the filtration of the infusible substance at 410.degree. C. and (c) a spinning temperature of as high as 400.degree.-420.degree. C. In a process disclosed in the specification of Japanese Patent Application Laid Open No. 118028/1975, a starting material is converted into a heavier fraction by heat treatment with stirring. According to examples given therein, a high softening point pitch is obtained by a simple step and a reaction for a long period of time and the removal of infusible matter is required for obtaining a low softening point pitch. A process disclosed in the specification of Japanese Patent Publication No. 7533/1978 comprises the polycondensation carried out in the presence of a Lewis acid catalyst such as aluminum chloride. However, this process is complicated and requires a great operational cost, since it also includes steps of removal of the catalyst and heat treatment before and after the catalyst removal. In a process disclosed in the specification of Japanese Patent Application Laid-Open No. 89635/1975, the polycondensation reaction of the non-mesophase pitch is carried out under heating until a mesophase content of 40% to 90% has been attained, while an inert gas is introduced in the liquid phase or under reduced pressure. A process disclosed in the specification of Japanese Patent Laid-Open No. 49125/1978 comprises carrying out the thermal polycondensation reaction under stirring until a mesophase content of 50% to 65% has been attained. In both of the pitches from above processes, mesophase is substantially equal to quinoline insoluble matter, and the softening point is controlled to the limit while a considerale non-mesophase is left. A disadvantage of the foregoing processes is that the spinning properties of the resultant pitch are poor, since the pitches are substantially heterogeneous. A process disclosed in the specification of Japanese Patent Publication No. 55625/1979 comprises the combination of the processes of said Japanese Patent Laid-Open No. 89635/1975 and said Patent Laid-Open No. 49125/1978; namely, this process comprises carrying out the polycondensation reaction by thermal cracking for a long period of time by the actions of bubbling of the inert gas stirring until 100% conversion into mesophase has been attained. This process has a problem in that the polycondensation reaction proceeds excessively to elevate both softening point and spinning temperature, though a homogeneous mesophase pitch can be obtained. A process disclosed in the specification of Japanese Patent Publication No. 160427/1979 includes a complicated, expensive process of extraction treatment with a solvent, and it has a problem that generally a mesophase pitch of a high softening point (above about 330.degree. C.) is formed, through the mesophase pitch is substantially homogeneous.
As will be understood from the above descriptions, it is difficult to produce a homogeneous mesophase pitch having a sufficiently low softening point and capable of being spun stably on a commercial scale by the conventional processes excluding catalytic processes. More particularly, according to the conventional processes, the thermal cracking/polycondensation reaction of the heavy hydrocarbons is carried out substantially in a simple step at a temperature of about 400.degree. C. over a long period of time. Therefore, as the mesophase content is increased gradually, the softening point of the pitch as a whole is elevated and, accordingly, temperature suitable for the melt spinning thereof (spinning temperature) is also elevated. If the reaction is terminated when a suitable spinning temperature has been attained, a heterogeneous pitch comprising an apparent mixture of the mesophase and the non-mesophase is formed, whereby the smooth spinning becomes impossible in many cases. This problem can be solved by continuing the reaction at a lower temperature to obtain a homogeneous pitch having a mesophase pitch content of essentially 100%. However, in this process, a long period of time is required for the reaction under strictly controlled temperature, and it is difficult to obtain a pitch of a high quality with a high reproducibility. Further, generally the softening point is extremely high in such a case and the stable spinning on a commercial basis is difficult. As a result, it is not easy to produce carbon fibers of a high performance.
After intensive experiments, the inventors have hit on the idea that the above problems in the prior art are due to the fact that the mesophase-constituting molecules are further subjected to the polycondensation reaction in the mesophase to make the molecular weight thereof excessively large, since the mesophase formed in the initial stage in the thermal cracking/polycondensation reactor is also kept at a high temperature until the completion of the reaction. The inventors have found that those defects of the conventional processes can be overcome by separating out the mesophase in the course of the thermal cracking/polycondensation reaction and that a pitch comprising nearly 100% mesophase and having a sufficiently low softening point can be obtained by this process. As means of separating the mesophase in the course of the thermal reaction, the following processes were tested:
(1) A process wherein the mesophase is concentrated by the extraction with a solvent such as n-heptane, benzene or toluene before it is separated out, and
(2) A process wherein the mesophase is separated out directly without using any solvent.
As a result, it has been found that the latter is superior to the former, since in the former, it is difficult to control the softening point of the mesophase and the steps are complicated. The present invention has been attained employing the latter process. Indeed the inventors have made intensive investigation of the latter process. For example, if a heavy hydrocarbon is subjected to thermal cracking/polycondensation reaction in an ordinary manner and the thermal reaction is suspended when the mesophase is formed partially, such as in the form of small spheres dispersed therein, and then the reaction product is allowed to stand and settle at a lower temperature, for example, in a temperature range at which the thermal cracking/polycondensation hardly occurs and the pitch is maintained sufficiently fluid, the small mesophase spheres precipitate and grow and form a coalescence in the reactor. These spheres are further coalesced at the bottom of the reactor and the reaction product is, therefore, divided clearly into an upper layer and a lower layer similar to that observed when water and oil settle in a vessel. The upper layer was taken out and examined to reveal that it was a non-mesophase pitch portion containing a small amount of fine spherical mesophase particles. The lower layer was nearly 100% mesophase pitch portion of a low softening point which could not have easily been obtained in the prior art. The lower layer pitch had excellent spinning properties and was molded and converted into carbon fibers by a conventional method which proved to be so-called high performance carbon fibers.
Therefore, the principal object of the present invention is to provide a process for producing a mesophase pitch wherein the whole steps can be completed in a short time of, for example, about 1-3 hours without necessitating complicated steps of high temperature filtration of infusible matter, extraction with a solvent and addition and removal of a catalyst.
Another object of the present invention is to provide a process for producing a mesophase pitch having a mesophase content of about 90%-100% and a low softening point (for example, 260.degree. C.) and, therefore, a low optimum spinning temperature (for example, 340.degree. C.).
Still another object of the present invention is to provide a process for producing a homogeneous mesophase pitch free of quality degradation which can be spun at a temperature far lower than a temperature at which remarkable thermal cracking/polycondensation reaction occurs (about 400.degree. C.) to form a carbon fiber product of a stable quality having excellent spinning properties (such as breakage frequency, fineness of the filament and filament diameter distribution).
Still another object of the present invention is to provide a process for producing mesophase pitch which does not substantially form any decomposition gases or infusible matter during the spinning, thereby producing pitch fibers scarcely containing bubbles or solid contaminants, and hence providing carbon fibers of a high strength.
A further object of the present invention is to provide a pitch comprising nearly 100% of the mesophase having an excellent molecular orientation capable of forming a carbon fiber product having a high modulus of elasticity in which crystal orientation in the graphite structure in a direction of the filament axis is well developed.
Another object of the present invention is to provide a process for producing a mesophase pitch wherein properties and quality of the pitch can be controlled stably and easily by providing steps of accumulation again, and separation of the liquid crystalline pitch after the thermal cracking/polycondensation reaction step, even if properties of the starting material vary considerably, or even if the operation conditions in the preceding step are varied to some extent.
The process of the present invention for producing mesophase pitch is described below.